Several independent risk factors have been associated with cardiovascular disease. These include hypertension, increased fibrinogen levels, high levels of triglycerides, elevated LDL cholesterol, elevated total cholesterol, and low levels of HDL cholesterol. HMG CoA reductase inhibitors (“statins”) are useful for treating conditions characterized by high LDL-c levels. It has been shown that lowering LDL-c is not sufficient for reducing the risk of cardiovascular disease in some patients, particularly those with normal LDL-c levels. This population pool is identified by the independent risk factor of low HDL-c. The increased risk of cardiovascular disease associated with low HDL-c levels has not yet been successfully addressed by drug therapy (i.e. currently there are no drugs on the market that are useful for raising HDL-c). (Bisgaier, C. L.; Pape, M. E. Curr. Pharm. Des. 1998, 4, 53-70).
Syndrome X (including metabolic syndrome) is loosely defined as a collection of abnormalities including hyperinsulinemia, obesity, elevated levels of triglycerides, uric acid, fibrinogen, small dense LDL particles, and plasminogen activator inhibitor 1(PAI-1), and decreased levels of HDL-c.
NIDDM is described as insulin resistance which in turn causes anomalous glucose output and a decrease in glucose uptake by skeletal muscle. These factors eventually lead to impaired glucose tolerance (IGT) and hyperinsulinemia.
Peroxisome Proliferator Activated Receptors (PPARs) are ophan receptors belonging to the steroid/retinoid receptor superfamily of ligand-activated transcription factors. See, for example Willson T. M. and Wahli, W., Curr. Opin. Chem. Biol.(1997) Vol. 1, pp. 235-241and Willson T. M. et al., J. Med. Chem (2000) Vol. 43, pp. 527-549. The binding of agonist ligands to the receptor results in changes in the expression level of mRNA's encoded by PPAR target genes.
Three mammalian Peroxisome Proliferator-Activated Receptors have been isolated and termed PPAR-alpha, PPAR-gamma, and PPAR-delta (also known as NUC1or PPAR-beta). These PPARs regulate expression of target genes by binding to DNA sequence elements, termed PPAR response elements (PPRE). To date, PPRE's have been identified in the enhancers of a number of genes encoding proteins that regulate lipid metabolism suggesting that PPARs play a pivotal role in the adipogenic signaling cascade and lipid homeostasis (H. Keller and W. Wahli, Trends Endocrin. Met 291-296, 4(1993)).
It has now been reported that thiazolidinediones are potent and selective activators of PPAR-gamma and bind directly to the PPAR-gamma receptor (J. M. Lehmann et al., J. Biol. Chem. 12953-12956, 270(1995)), providing evidence that PPAR-gamma is a possible target for the therapeutic actions of the thiazolidinediones.
Activators of the nuclear receptor PPARγ, for example troglitazone, have been shown in the clinic to enhance insulin-action, reduce serum glucose and have small but significant effects on reducing serum triglyceride levels in patients with Type 2diabetes. See, for example, D. E. Kelly et al., Curr. Opin. Endocrinol. Diabetes, 90-96, 5(2), (1998); M. D. Johnson et al., Ann. Pharmacother., 337-348, 32(3), (1997); and M. Leutenegger et al., Curr. Ther. Res., 403-416, 58(7), (1997).
The mechanism for this triglyceride lowering effect appears to be predominantly increased clearance of very low density lipoproteins (VLDL) through induction of lipoprotein lipase (LPL) gene expression. See, for example, B. Staels et al., Arterioscler. Thromb., Vasc. Biol., 1756-1764, 17(9), (1997).
Fibrates are a class of drugs which may lower serum triglycerides 20-50%, lower LDLc 10-15%, shift the LDL particle size from the more atherogenic small dense to normal dense LDL, and increase HDLc 10-15%. Experimental evidence indicates that the effects of fibrates on serum lipids are mediated through activation of PPARα. See, for example, B. Staels et al., Curr. Pharm. Des., 1-14, 3(1), (1997). Activation of PPARα results in transcription of enzymes that increase fatty acid catabolism and decrease de-novo fatty acid synthesis in the liver resulting in decreased triglyceride synthesis and VLDL production/secretion. In addition, PPARα activation decreases production of apoC-III. Reduction in apoC-III, an inhibitor of LPL activity, increases clearance of VLDL. See, for example, J. Auwerx et al., Atherosclerosis, (Shannon, Irel.), S29-S37, 124(Suppl), (1996).
Certain compounds that activate or otherwise interact with one or more of the PPARs have been implicated in the regulation of triglyceride and cholesterol levels in animal models. See, for example, U.S. Pat. No. 5,847,008(Doebber et al.) and U.S. Pat No. 5,859,051(Adams et al.) and PCT publications WO 97/28149(Leibowitz et al.) and WO99/04815(Shimokawa et al.). In a recent report (Berger et al., J. Biol. Chem. 1999), Vol. 274, pp. 6718-6725) it was stated that PPARδ activation does not appear to modulate glucose or triglyceride levels.